Figure 1.

The neurogenic system in the adult crayfish brain. (A) Diagram of the crayfish brain. The soma clusters 9 and 10 (circled) flank two prominent neuropil regions of the deutocerebrum, the olfactory (OL) and accessory (AL) lobes. The OL is the primary olfactory processing area in the crustacean brain and is functionally equivalent to the olfactory bulb. The AL is a higher order processing area that integrates olfactory, visual and mechanosensory information [8,9]. (B) Serotonin immunostaining of a crayfish brain. Each dorsal giant neuron (DGN; arrow) innervates the ipsilateral OL and AL. The intense labeling of the OLs and ALs is due to the massive DGN projection to these areas. (cluster 9, cl9; cluster 10, cl10) (C) Confocal image of the ventral surface of the crayfish P. clarkii brain labeled immunocytochemically for 5-bromo-2-deoxyuridine (BrdU; green) and glutamine synthetase (blue), and counterstained with propidium iodide (red), a marker of nucleic acids. BrdU-labeled cells can be observed in the migratory streams (long arrows indicate direction of movement) and within both the lateral proliferation zone (LPZ) and medial proliferation zone (MPZ). The LPZ supplies cells to cluster 10, where they will differentiate into olfactory projection neurons. The MPZ supplies cells to cluster 9, where they will differentiate into local olfactory interneurons. (D) Model summarizing our current view of events leading to the production of new olfactory interneurons in adult crayfish. Neuronal precursor (first generation) cells exhibiting glial characteristics reside within a neurogenic niche where they divide symmetrically. Their daughters (second-generation precursors) migrate along tracts created by the fibers of the niche cells towards either the LPZ or the MPZ. At least one more division will occur in the LPZ and MPZ before the progeny (third- and subsequent-generations) differentiate into neurons. Scale bars: 200 μm (B); 100 μm (C). Panels (A, D) adapted from Sullivan et al. [11].